1. Field
Embodiments relate to a walking robot, respective joints of which are operated through torque servo control so as to achieve stable pose control, and a pose control method thereof.
2. Description of the Related Art
In general, research and development of walking robots which have a joint system similar to that of humans and coexist with humans in human working and living spaces is actively progressing. The walking robots are multi-legged walking robots having a plurality of legs, such as two or three legs or more. In order to achieve stable walking of the robot, actuators, such as electric motors or hydraulic motors, located at respective joints of the robot need to be driven. As a method to drive the actuators, there is a position-based Zero Moment Point (hereinafter, referred to as ZMP) control method in which command angles of respective joints, i.e., command positions, are given and the joints are controlled so as to track the command positions. As an additional method to drive the actuators, there is a torque-based Finite State Machine (hereinafter, referred to as FSM) control method in which command torques of respective joints are given and the joints are controlled so as to track the command torques.
In the ZMP control method, walking direction, walking stride, and walking velocity of a robot are set in advance so as to satisfy a ZMP constraint. For example, a ZMP constraint may be a condition that a ZMP is present in a safety region within a support polygon formed by supporting leg(s) (if the robot is supported by one leg, this means the region of the leg, and if the robot is supported by two legs, this means a region set to have a small area within a convex polygon including the regions of the two legs in consideration of safety). In the ZMP control method, walking patterns of the respective legs corresponding to the set factors are generated, and walking trajectories of the respective legs are calculated based on the walking patterns. Further, angles of joints of the respective legs are calculated through inverse kinematics of the calculated walking trajectories, and target control values of the respective joints are calculated based on current angles and target angles of the respective joints. Moreover, servo control allowing the respective legs to track the calculated walking trajectories per control time is carried out. That is, during walking of the robot, whether or not positions of the respective joints precisely track the walking trajectories according to the walking patterns is detected, and if it is detected that the respective legs deviate from the walking trajectories, torques of the motors are adjusted so that the respective legs precisely track the walking trajectories.
However, the ZMP control method is a position-based control method and thus achieves precise position control, but needs to perform precise angle control of the respective joints in order to control the ZMP and thus requires a high servo gain. Thereby, the ZMP control method requires high current and thus has low energy efficiency and high stiffness of the joints, thereby applying high impact to a surrounding environment. Further, the ZMP control method needs to avoid kinematic singularities in order to calculate angles of the respective joints, thereby causing the robot to take a pose with knees bent at all times during walking and thus to have an unnatural gait differing from that of a human.